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  • Air–Sea Interactions from the Diurnal to the Intraseasonal during the PISTON, MISOBOB, and CAMP2Ex Observational Campaigns in the Tropics x
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K. Jossia Joseph, Amit Tandon, R. Venkatesan, J. Thomas Farrar, and Robert A. Weller

Abstract

The inception of a moored buoy network in the northern Indian Ocean in 1997 paved the way for systematic collection of long-term time series observations of meteorological and oceanographic parameters. This buoy network was revamped in 2011 with Ocean Moored buoy Network for north Indian Ocean (OMNI) buoys fitted with additional sensors to better quantify the air–sea fluxes. An intercomparison of OMNI buoy measurements with the nearby Woods Hole Oceanographic Institution (WHOI) mooring during the year 2015 revealed an overestimation of downwelling longwave radiation (LWR↓). Analysis of the OMNI and WHOI radiation sensors at a test station at National Institute of Ocean Technology (NIOT) during 2019 revealed that the accurate and stable amplification of the thermopile voltage records along with the customized datalogger in the WHOI system results in better estimations of LWR↓. The offset in NIOT measured LWR↓ is estimated first by segregating the LWR↓ during clear-sky conditions identified using the downwelling shortwave radiation measurements from the same test station, and second, finding the offset by taking the difference with expected theoretical clear-sky LWR↓. The corrected LWR↓ exhibited good agreement with that of collocated WHOI measurements, with a correlation of 0.93. This method is applied to the OMNI field measurements and again compared with the nearby WHOI mooring measurements, exhibiting a better correlation of 0.95. This work has led to the revamping of radiation measurements in OMNI buoys and provides a reliable method to correct past measurements and improve estimation of air–sea fluxes in the Indian Ocean.

Significance Statement

Downwelling longwave radiation (LWR↓) is an important climate variable for calculating air–sea heat exchange and quantifying Earth’s energy budget. An intercomparison of LWR↓ measurements between ocean observing platforms in the north Indian Ocean revealed a systematic offset in National Institute of Ocean Technology (NIOT) Ocean Moored buoy Network for north Indian Ocean (OMNI) buoys. The observed offset limited our capability to accurately estimate air–sea fluxes in the Indian Ocean. The sensor measurements were compared with a standard reference system, which revealed problems in thermopile amplifier as the root cause of the offset. This work led to the development of a reliable method to correct the offset in LWR↓ and revamping of radiation measurements in NIOT-OMNI buoys. The correction is being applied to the past measurements from 12 OMNI buoys over 8 years to improve the estimation of air–sea fluxes in the Indian Ocean.

Open access
Emily Shroyer, Amit Tandon, Debasis Sengupta, Harindra J. S. Fernando, Andrew J. Lucas, J. Thomas Farrar, Rajib Chattopadhyay, Simon de Szoeke, Maria Flatau, Adam Rydbeck, Hemantha Wijesekera, Michael McPhaden, Hyodae Seo, Aneesh Subramanian, R Venkatesan, Jossia Joseph, S. Ramsundaram, Arnold L. Gordon, Shannon M. Bohman, Jaynise Pérez, Iury T. Simoes-Sousa, Steven R. Jayne, Robert E. Todd, G. S. Bhat, Matthias Lankhorst, Tamara Schlosser, Katherine Adams, S. U. P Jinadasa, Manikandan Mathur, M. Mohapatra, E. Pattabhi Rama Rao, A. K. Sahai, Rashmi Sharma, Craig Lee, Luc Rainville, Deepak Cherian, Kerstin Cullen, Luca R. Centurioni, Verena Hormann, Jennifer MacKinnon, Uwe Send, Arachaporn Anutaliya, Amy Waterhouse, Garrett S. Black, Jeremy A. Dehart, Kaitlyn M. Woods, Edward Creegan, Gad Levy, Lakshmi H. Kantha, and Bulusu Subrahmanyam

Abstract

In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air–sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the United States, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air–sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ∼20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10–12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ∼20 to 50 m), cooling SST (by ∼1°C), and warming/drying of the lower to midtroposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air–sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.

Full access
Adam H. Sobel, Janet Sprintall, Eric D. Maloney, Zane K. Martin, Shuguang Wang, Simon P. de Szoeke, Benjamin C. Trabing, and Steven A. Rutledge

Abstract

The Propagation of Intraseasonal Tropical Oscillations (PISTON) experiment conducted a field campaign in August–October 2018. The R/V Thomas G. Thompson made two cruises in the western North Pacific region north of Palau and east of the Philippines. Using select field observations and global observational and reanalysis datasets, this study describes the large-scale state and evolution of the atmosphere and ocean during these cruises. Intraseasonal variability was weak during the field program, except for a period of suppressed convection in October. Tropical cyclone activity, on the other hand, was strong. Variability at the ship location was characterized by periods of low-level easterly atmospheric flow with embedded westward propagating synoptic-scale atmospheric disturbances, punctuated by periods of strong low-level westerly winds that were both connected to the Asian monsoon westerlies and associated with tropical cyclones. In the most dramatic case, westerlies persisted for days during and after tropical cyclone Jebi had passed to the north of the ship. In these periods, the sea surface temperature was reduced by a couple of degrees by both wind mixing and net surface heat fluxes that were strongly (~200 W m−2) out of the ocean, due to both large latent heat flux and cloud shading associated with widespread deep convection. Underway conductivity–temperature transects showed dramatic cooling and deepening of the ocean mixed layer and erosion of the barrier layer after the passage of Typhoon Mangkhut due to entrainment of cooler water from below. Strong zonal currents observed over at least the upper 400 m were likely related to the generation and propagation of near-inertial currents.

Full access
Emily Shroyer, Amit Tandon, Debasis Sengupta, Harindra J.S. Fernando, Andrew J. Lucas, J. Thomas Farrar, Rajib Chattopadhyay, Simon de Szoeke, Maria Flatau, Adam Rydbeck, Hemantha Wijesekera, Michael McPhaden, Hyodae Seo, Aneesh Subramanian, R Venkatesan, Jossia Joseph, S. Ramsundaram, Arnold L. Gordon, Shannon M. Bohman, Jaynise Pérez, Iury T. Simoes-Sousa, Steven R. Jayne, Robert E. Todd, G.S. Bhat, Matthias Lankhorst, Tamara Schlosser, Katherine Adams, S.U.P Jinadasa, Manikandan Mathur, M. Mohapatra, E. Pattabhi Rama Rao, A. K. Sahai, Rashmi Sharma, Craig Lee, Luc Rainville, Deepak Cherian, Kerstin Cullen, Luca R. Centurioni, Verena Hormann, Jennifer MacKinnon, Uwe Send, Arachaporn Anutaliya, Amy Waterhouse, Garrett S. Black, Jeremy A. Dehart, Kaitlyn M. Woods, Edward Creegan, Gad Levy, Lakshmi H Kantha, and Bulusu Subrahmanyam

Abstract

In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air-sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the US, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air-sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ~20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10−12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ~20 to 50 m), cooling SST (by ~ 1°C), and warming/drying of the lower to mid-troposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air-sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.

Full access
Simon P. de Szoeke

Abstract

A small integrated oceanographic thermometer with a nominal response time of 1 s was affixed to a floating hose “sea snake” towed near the bow of a research vessel. The sensor measured the near-surface ocean temperature accurately and in agreement with other platforms. The effect of conduction and evaporation is modeled for a sensor impulsively alternated between water and air. Large thermal mass makes most sea snake thermometers insensitive to temperature impulses. The smaller 1-s thermometer cooled by evaporation, but the sensor never reached the wet-bulb temperature. The cooling was less than 6% of the (~2.7°C) difference between the ocean temperature and the wet-bulb temperature in 99% of 2-s−1 samples. Filtering outliers, such as with a median, effectively removes the evaporative cooling effect from 1- or 10-min average temperatures.

Open access
Kenneth G. Hughes, James N. Moum, and Emily L. Shroyer

Abstract

Penetration of solar radiation in the upper few meters of the ocean creates a near-surface, stratified diurnal warm layer. Wind stress accelerates a diurnal jet in this layer. Turbulence generated at the diurnal thermocline, where the shear of the diurnal jet is concentrated, redistributes heat downward via mixing. New measurements of temperature and turbulence from fast thermistors on a surface-following platform depict the details of this sequence in both time and depth. Temporally, the sequence at a fixed depth follows a counterclockwise path in logϵ–logN parameter space. This path also captures the evolution of buoyancy Reynolds number (a proxy for the anisotropy of the turbulence) and Ozmidov scale (a proxy for the outer vertical length scale of turbulence in the absence of the free surface). Vertically, the solar heat flux always leads to heating of fluid parcels in the upper few meters, whereas the turbulent heat flux divergence changes sign across the depth of maximum vertical temperature gradient, cooling fluid parcels above and heating fluid parcels below. In general, our measurements of fluid parcel heating or cooling rates of order 0.1°C h−1 are consistent with our estimates of heat flux divergence. In weak winds (<2 m s−1), sea surface temperature (SST) is controlled by the depth-dependent absorption of solar radiation. In stronger winds, turbulent mixing controls SST.

Free access
Jai Sukhatme, Dipanjan Chaudhuri, Jennifer MacKinnon, S. Shivaprasad, and Debasis Sengupta

Abstract

Horizontal currents in the Bay of Bengal were measured on eight cruises covering a total of 8600 km using a 300-kHz acoustic Doppler current profiler (ADCP). The cruises are distributed over multiple seasons and regions of the Bay. Horizontal wavenumber spectra of these currents over depths of 12–54 m and wavelengths from 2 to 400 km were decomposed into rotational and divergent components assuming isotropy. An average of across- and along-track spectra over all cruises shows that the spectral slope of horizontal kinetic energy for wavelengths of 10–80-km scales with an exponent of −1.7 ± 0.05, which transitions to a steeper slope for wavelengths above 80 km. The rotational component is significantly larger than the divergent component at scales greater than 80 km, while the ratio of the two is nearly constant with a mean of 1.16 ± 0.4 between 10 and 80 km. The measurements show a fair amount of variability and spectral levels vary between cruises by about a factor of 5 over 10–100 km. Velocity differences over 10–80 km show probability density functions and structure functions with stretched exponential behavior and anomalous scaling. Comparisons with the Garrett–Munk internal wave spectrum indicate that inertia–gravity waves account for only a modest fraction of the kinetic energy between 10 and 80 km. These constraints suggest that the near-surface flow in the Bay is primarily balanced and follows a forward enstrophy transfer quasigeostrophic regime for wavelengths greater than approximately 80 km, with a larger role for unbalanced rotating stratified turbulence and internal waves at smaller scales.

Free access
Kyle Chudler, Weixin Xu, and Steven A. Rutledge

Abstract

During the boreal summer, satellite-based precipitation estimates indicate a distinct maximum in rainfall off the west coast of the island of Luzon in the Philippines. Also occurring during the summer months is the boreal summer intraseasonal oscillation (BSISO), a main driver of intraseasonal variability in the region. This study investigates the diurnal variability of convective intensity, morphology, and precipitation coverage offshore and over the island of Luzon. The results are then composited by BSISO activity. Results of this study indicate that offshore precipitation is markedly increased during active BSISO phases, when strong low-level southwesterly monsoon winds bring increased moisture and enhanced convergence upwind of the island’s high terrain. A key finding of this work is the existence of an afternoon maximum in convection over Luzon even during active BSISO phases, when solar heating and instability are apparently reduced due to enhanced cloud cover. This result is important, as previous studies have shown in other areas of the tropics afternoon convection over landmasses is a key component to offshore precipitation. Although offshore precipitation is maximized in the evening hours during active phases, results indicate that precipitation frequently occurs over the ocean around the clock (both as organized systems and isolated, shallow showers), possibly owing to an increase in sensible and latent heat fluxes, vertical wind shear, and convergence of the monsoon flow with land features.

Free access
Benjamin A. Toms, Susan C. van den Heever, Emily M. Riley Dellaripa, Stephen M. Saleeby, and Eric D. Maloney

Abstract

While the boreal summer Madden–Julian oscillation (MJO) is commonly defined as a planetary-scale disturbance, the convective elements that constitute its cloud dipole exhibit pronounced variability in their morphology. We therefore investigate the relationship between the intraseasonal cloud anomaly of the MJO and the convective elements that populate its interior by simulating a boreal summer MJO event over the Maritime Continent using a cloud-resolving model. A progressive relationship between convective cell morphology and the MJO within the convectively enhanced region of the MJO was identified and characterized as follows: anomalously long-lasting cells in the initial phases, followed by an increased number of cells in the intermediate phases, progressing into more expansive cells in the terminal phases. A progressive relationship does not seem to exist within the convectively suppressed region of the MJO within the simulated domain, however. Within the convectively enhanced region of the MJO, the progressive relationship is partially explained by the evolution of bulk atmospheric characteristics, such as instability and wind shear. Positive midlevel moisture anomalies coincide with anomalously long-lasting convective cells, which is hypothesized to further cascade into an increase in convective cell volume, although variability in the number of convective cells seems to be related to an unidentified variable. This intraseasonal relationship between convective cell morphology and the boreal summer MJO within the Maritime Continent may have broader implications for the large-scale structure and evolution of the MJO, related to both convective moistening and cloud-radiative feedbacks.

Free access
Kenneth G. Hughes, James N. Moum, and Emily L. Shroyer

Abstract

The daily formation of near-surface ocean stratification caused by penetrating solar radiation modifies heat fluxes through the air–sea interface, turbulence dissipation in the mixed layer, and the vertical profile of lateral transport. The transport is altered because momentum from wind is trapped in a thin near-surface layer, the diurnal warm layer. We investigate the dynamics of this layer, with particular attention to the vertical shear of horizontal velocity. We first develop a quantitative link between the near-surface shear components that relates the crosswind component to the inertial turning of the along-wind component. Three days of high-resolution velocity observations confirm this relation. Clear colocation of shear and stratification with Richardson numbers near 0.25 indicate marginal instability. Idealized numerical modeling is then invoked to extrapolate below the observed wind speeds. This modeling, together with a simple energetic scaling analysis, provides a rule of thumb that the diurnal shear evolves differently above and below a 2 m s−1 wind speed, with limited sensitivity of this threshold to latitude and mean net surface heat flux. Only above this wind speed is the energy input sufficient to overcome the stabilizing buoyancy flux and thereby induce marginal instability. The differing shear regimes explain differences in the timing and magnitude of diurnal sea surface temperature anomalies.

Free access